Heat treatment of steel



July 7, 1959 G. A. ROBERTS 7 ET AL 2,893,902

HEAT TREATMENT OF STEEL 2 Sheets-Sheet 1 Original Filed Aug. 20,

ROBERTS HAMAKER, J'R.

AVG/0V5 GEORGE A.

July 7, T959 (5, RO ET AL 2,893,902

HEAT TREATMENT OF STEEL 1 Filed Aug. 20, 1957 2 Sheets-Sheet 2 Origina GEORGE A. ROBERTS JOHN c. HAMAKER,JR. BYK

ATTORNEY.

HEAT TREATWNT OF STEEL George A. Roberts and John C. Hamaker, .ha, Latrobe,

Pa., assignors to Vanadium-Alloys Steel Company, Latrobe, Pa.

Continuation of abandoned application Serial No. 679,227, August 20, 1957. This application February 4, 1959, Serial No. 798,797

10 Claims. (Cl. 148-12) In our co-pending application, Serial No. 679,231, filed on August 20, 1957, we have disclosed a new and improved alloy steel composition having ultra high strength characteristics at very high temperatures. The present invention relates to a novel method of heat treating our own new composition as well as other ferrous materials which are also capable of air hardening.

This application is a continuation of co-pending application, Serial No. 679,227, filed on August 20, 1957, now abandoned.

One of the main objects of our improved heat treatment is to improve the mechanical properties of the material being treated in the transverse direction, which is to say, that direction which lies perpendicular to the flow lines in the steel, formed by hot working, such as forging or rolling. The transverse properties of ultra high strength steels are of major concern to, and are carefully selected and specified by, for instance, aircraft designers and manufacturers in the employment of billets or forgings of such material in structural parts and components such as landing gear, engine mounts, structural members, and the like.

Another object of our invention is to provide a special heat treatment which will impart to the metal transverse properties that surpass values that have thus far been reported.

As disclosed in our co-pending application, Serial No. 679,231 the ultra high strength, high temperature steels thereof have the following analysis:

According to our invention materials of this class, which are capable of air hardening, such, for example, as. tool steels containing approximately chromium, may be heat treated in the manner now to be described.

First the material is hot worked in a suitable manner, such as billet or die forging, upsetting or hot rolling. This operation is ultimately followed by a standard annealing operation to condition the steel for machinability, and subsequent hardening and tempering, as disclosed in our co-pending case, for developing ultra high strength qualities.

Between the steps or operations. of (a) hot working and (b) annealing, we include the following treatments:

(1) To even out the grain structure we employ high temperature homogenization, consisting of heating the material to 19002050 F. for 2 to 4 hours, followed by cooling in an insulating mediunnfor example, by burying in ashes, or by fast furnace cooling. Temperatures above 2050 F. should generally; be avoided because rapid grain 2,893,902 Patented July 7, 1959 growth may occur. At temperatures of 2050" F., and below, the fine forged grain is retained.

(2) To eliminate hard spots and to produce ultra high strength, the material is normalized at low temperatures, which will generally consist in heating the steel to a soaking temperature of 1600-1700" F-., holding the material at the heated temperature for /2 to 1 /2 hours, and subsequently air cooling it.

Our foregoing step of normalizingsteels of this general class is actually contrary to the teachings of the prior art which are to the elfect that such steels should not be normalized.

The exact temperatures and time periods given above have specific application to ourown improved ultra high strength material also recited: above. Inasmuch as our heat treatment is also beneficial for other ultra high strength steels it will be understood that essentially our contribution to the art consists in the discovery that the properties or physicals of steels orthe class of, let us say, tool steels, may be heat treated tov develop. ultra high strength characteristics at room or very. high temperatures by (a) homogenization and (b) normalization. of the material between hot working and final; annealing.

The American Iron and Steel Institute (AISI) Steel Products Manual, Tool Steels, cautions against normalizing steels of this type as does also the American Society of Metals (ASM), 1948, Metalsv Handbook (Type lVB), page 657, and the SAE Handbook (Type H11, H12 and H13) which are the leading references on such materials. In general, normalizing, according: to these authorities is not recommended for any air hardeningsteel, as stated on page 654 of the 1948 ASM metals handbook. Our material, described in our co-pending application, is one of the most deeply air hardening steels ever discovered.

Although high temperature homogenization has been used on castings and low alloy steels, it has never been reported as suitable for medium to high alloy steels, the types with which we are here concerned. The principal reasons for this, it is theorized, are the slow, sluggish, diffusion of the high percentage of alloying elements and the presence in the high: alloy steels of carbides which frequently resist solution to the melting point of the steel and therefore resist redistribution by a homogenizing treatment.

The application and effects of our improved heat treatment are described in the following paragraphs and shown in the accompanying drawings.

Turning to the drawings, Figures; 1 and 2 illustrate the effect of high temperature homogenization (2 hours at 2000 F., followed by an air cool) on the microstnlcture within one and one-half inch of the center of'afifteen and one-half inch square billet of our material. Figure 1 shows the structure after the usual anneal, depicting poor carbide distribution and carbide structures which deteriorate transverse. properties (500x magnification).

These carbide structures or chains lying in the grain boundaries are formed during. cooling or annealing of a billet. They form lines of discontinuity which are detrimental to transverse ductility. Figure 2 shows the iden tical material after a high temperature homogenization treatment at 2000 F., followed by annealing to place it in the same soft condition (180 Brinell hardness) as the structure of Figure 1. The pronounced refinement and improved carbide distribution in the. homogenized structure should be favorably observed;

Figures 3 and 4 picture the effect of the low temperature normalizing treatment on the fracture appearance of a specimen of our material. Figure 3 shows the structure following the usual anneal after heat treatment and Figure 4 the refined and improved structure that results from the normalizing treatment followed by an. anneal.

Figures 5 and 6 show the effect of thelow temperature normalizing treatment (air cooled from 1650" F.) on the microstructure of our material 5 magnification). Figure shows the structure following the usual anneal after hot working and thereby corresponds to the specimen illustrated in Figure 3. Figure 6 shows the improved and more uniform grain structure which results from the low temperature normalization treatment followed by an anneal.

Thus the homogenizing treatment, which is particularly advantageous for heavy sections, for example 2" and larger results in uniformity of grain structure and excellent distribution of fine carbides. The normalizing treatment eliminates hard spots, results in still greater uniformity of annealed and heat treated hardness and high strength properties.

The table below shows the marked improvement in tensile properties which results from treating the billets according to our improved process. Of particular importance are the values for transverse reduction of area, a major criterion of many aircraft specifications. In a 15 /2" square billet at mid-radius (midway between center and surface) our process of heat treatment eifected a 28% improvement in transverse reduction of area and raised the lowest value in any single test from 9.2 to 13.2. And this was accomplished despite a slightly diiferent heat treatment to increase the tensile and yield strengths in the second test-a factor that usually decreases rather than increases ductility. In the center of an 8" square billet the improvement was even more pronounced, providing an increase of 38%, with the minimum raised from 9.0 to 13.4. Again, this was accomplished despite a change in heat treatment to provide 10,000 p.s.i. higher tensile strength. At mid-radius of a 7 square billet the transverse reduction of area values are exceptionally high giving an average of 25.3% with a minimum of 23.7%:

TABLE material and, intermediate of such hot working and subsequent annealing steps, submitting the material to a homogenization treatment.

3. An improved process of heat treating air hardening martensitic structure steels to improve the transverse mechanical properties thereof which comprises the steps of initially hot working and subsequently annealing the material and, intermediate of such hot working and subsequent annealing steps, submitting the material to a low temperature normalizing treatment.

4. An improved process of heat treating an alloy steel composition consisting essentially of from about .20 to about .50% carbon, from about .10 to about 1.50% silicon, from about .10 to about 1.00% manganese, from about 3.00 to about 8.00% chromium, from about .50 to about 3.00% molybdenum, from about .20 to about 1.25% vanadium and the remainder principally iron, which comprises the steps of initially hot working and subsequently annealing the composition and, intermediate of such hot working and subsequent annealing steps, submitting the composition to a homogenization treatment followed by a low temperature normalizing treatment.

5. An improved process of heat treating an alloy steel composition consisting essentially of from about .20 to about .50% carbon, from about .10 to about 1.50% silicon, from about .10 to about 1.00% manganese, from about 3.00 to about 8.00% chromium, from about .50 to about 3.00% molybdenum, from about .20 to about 1.25% vanadium and the remainder principally iron, which comprises the steps of initially hot working and subsequently annealing the composition and, intermediate of such hot working and subsequent annealing steps, submitting the composition to a homogenization treatment.

6. An improved process of heat treating an alloy steel composition consisting essentially of from about .20 to Improvement in transverse ductility of high tensile specimens from large' Vascojet 1000 billets provided by special homogenizing and normalizing treatment [0.505" specimens, cut transversely ironliogil let, austenitized 30 minutes at 1850 F., air cooled, and tempered 2+2+2 hours at 1015- 13. Three to eight tests per condition] Specimen Reduction of Area, Specimen Billet Hard- 0.2% Yield Percent Billet Size Location Treatment ness, Strength, Tensile Elongation, Percent Re p.s.i. Strength,

p.s.i.

Average Range Average Range Midradius 1 A 49. 6 208, 000 250, 000 5. 6 5. 0-6. 8 12. 9 9. 2-17. 5 Midradiua... H, N, A 49. 6 212, 000 256,000 6. 3 5. 3-7. 0 16. 5 13. 2-20. 5 Center :i: 1"- A 50. 3 211, 000 5, 000 4. 9 4. 3-6. 3 10. 7 9. 0-12. 8 Center 5: 1"- H, N, A 51. 4 221,000 265, 000 5. 4 4. 5-6.1 14. 8 13. 4-16. 9 Midradlus I H, N, A 51. 4 223, 000 266, 000 6. 9 6. 3-7. 5 25. 3 23. 7-28. 2

Annealed only-usual for this steel.

It will be understood that we prefer to employ both the homogenizing and normalizing steps intermediate hot working and final annealing in order to obtain the best results. 2 However, dropping one of the intermediate treatments while retaining .the other may fall within the scope of our invention as may be determined from the following claims.

We claim: v '1. An improved process of heat treating air hardening martensitic structure I steels to improve the transverse mechanical properties thereof which comprises the steps of initially hot working and subsequently annealing the material and, intermediate of such hot working and subsequent annealing steps, submitting the material to a homogenization treatment followed by a low temperature normalizing treatment. 1

2. An improved process of heat treating air hardening martensitic structure steels to improve the transverse mechanical properties thereof which comprises the steps of initially hot working and subsequently annealing the about .50% carbon, from about .10 to about 1.50% silicon, from about .10 to about 1.00% manganese, from about 3.00 to about 8.00% chromium, from about .50 to about 3.00% molybdenum, from about .20 to about 1.25% vanadium and the remainder principally iron, which comprises the steps of initially hot working and subsequently annealing the composition and, intermediate of such hot working and subsequent annealing steps, submitting the composition to a low temperature normalizing treatment.

7. An improved process of heat treating an alloy steel composition consisting essentially of from about .20 to about .50% carbon, from about-.10 to about 1.50% silicon, from about .10 to about 1.00% manganese, from about 3.00 to about 8.00% chromium, from about .50 to about 3.00% molybdenum, from about .20 to about 1.25% vanadium and the remainder principally iron, which comprises the steps of initially hot working and subsequently annealing the composition and, intermediat? of such hot working and subsequent annealing steps,

M in

heating the composition to 19002050 F. for 2 to 4 hours and thereafter cooling said material.

8. An improved process of heat treating an alloy steel composition consisting essentially of from about .20 to about 50% carbon, from about .10 to about 1.50% silicon, from about .10 to about 1.00% manganese, from about 3.00 to about 8.00% chromium, from about .50 to about 3.00% molybdenum, from about .20 to about 1.25% vanadium and the remainder principally iron, which comprises the steps of initially hot working and subsequently annealing the composition and, intermediate of such hot working and subsequent annealing steps, heating the composition to a temperature of l600-1700 F. for /2 to 1 /2 hours and subsequently cooling said composition.

9. An improved process of heat treating an alloy steel composition consisting essentially of from about .20 to about .50% carbon, from about .10 to about 1.50% silicon, from about .10 to about 1.00% manganese, from about 3.00 to about 8.00% chromium, from about .50 20 to about 3.00% molybdenum, from about .20 to about 1.25% vanadium and the remainder principally iron,

which comprises the steps of initially hot working and subsequently annealing the composition and, intermediate of such hot working and subsequent annealing steps, heating the composition to 1900-2050 F. for 2 to 4 hours, thereafter cooling it, and subsequently again heating the composition to a temperature of 16001700 F. for /2 to 1 /2 hours, and again cooling said material.

10. An ultrahigh strength alloy steel article consisting essentially of from about .20% to about 50% carbon, about .10% to about 1.50% silicon, about .10% to about 1.00% manganese, about 3.00% to about 8.00% chromium, about 50% to about 3.00% molybdenum, about .20% to about 1.25% vanadium, and the remainder principally iron, said alloy steel having been initially hot worked and subsequently annealed and, intermediate of said hot working and subsequent annealing, submitted to a homogenization treatment followed by a normalizing treatment.

References Cited in the file of this patent Metals Handbook, 1948 edition, published by ASM. Pages 656 and 657. 

1. AN IMPROVED PROCESS OF HEAT TREATING AIR HARDENING MARTENSITIC STRUCTURE STEELS TO IMPROVE THE TRANSVERSE MECHANICAL PROPERTIES THEREOF WHICH COMPRISES THE STEPS OF INITIALLY HOT WORKING AND SUBSEQUENTLY ANNEALING THE MATERIAL AND, INTERMEDIATE OF SUCH HOT WORKING AND SUBSEQUENT ANNEALING STEPS, SUBMITTING THE MATERIAL TO A HOMOGENIZATION TREATMENT FOLLOWED BY A LOW TEMPERATURE NORMALIZING TREATMENT. 